The technical development of directly injected gasoline and diesel engines operated with a lean mixture has led to a reduction in fuel consumption in recent years. As a result of this trend, the CO2 emission from mobile sources can in future be permanently reduced without losses of power and mineral oil resources can be saved. The disadvantage of the operation of internal combustion engines using a lean mixture is however the formation of oxides of nitrogen (NOx) owing to the large oxygen excess in the combustion chamber. Typically, the NOx concentrations in the engine exhaust gas are a few hundred ppm. The future European exhaust gas standards limit the amount of NOx emitted per kilometer driven to 0.5 g/km from Jan. 1, 2001 and to 0.25 g/km from Jan. 1, 2005. The use of directly injected engines using a lean mixture in car and truck traffic therefore requires efficient removal of oxides of nitrogen from the combustion exhaust gases.
Various possibilities are available for complying with the emission limits:
The technical measures relating to the engine include, for example, exhaust gas recycling to the engine (EGR). The NOx emission can be very efficiently reduced by this measure without losses of motor power since it results in a decrease in the oxygen content in the combustion chamber and hence suppression of the combustion of atmospheric nitrogen. The disadvantage of exhaust gas recycling is the simultaneous increase in the emission of soot particles. This state of affairs referred to as the diesel dilemma means that in practice it is not possible to comply with both limits—NOx as well as soot emission—by means of exhaust gas recycling alone. The effect of EGR apparatuses is partly improved by selective recycling of the oxides of nitrogen. The oxides of nitrogen are temporarily stored and are metered in concentrated form into the intake air of the engine. However, technical measures relating to the engine are not sufficient for achieving the required removal of oxides of nitrogen.
Improved removal of oxides of nitrogen from exhaust gas can be achieved in particular by exhaust gas catalysts in whose active materials the oxides of nitrogen are reacted with a reducing agent. Such catalyst systems rarely act through simple decomposition of the oxides of nitrogen into nitrogen. More frequently, fuel (diesel or gasoline fuel) is used as the reducing agent. The active materials used are as a rule based on noble metals which are applied to an oxidic support material and are arranged as a coating in a molding having a low pressure drop in the exhaust gas line of the vehicle (cf. for example WO 98/40153). In practice, a plurality of catalysts whose temperature ranges for the removal of oxides of nitrogen from the exhaust gas and for the oxidation of uncombusted exhaust gas components are each shifted relative to one another are used in such an exhaust gas system in the case of diesel vehicles. The broadened temperature window results in improved performance of the catalyst arrangement over the entire operating range (urban and nonurban). It is therefore necessary, particularly for diesel vehicles, to mount a catalytic converter close to the engine so that the heat contained in the exhaust gas can heat up the catalyst rapidly and directly and the catalyst rapidly reaches its active temperature.
WO 98/40153, too, describes such a system consisting of two catalysts based on noble metals. At conversions of ≧80% of the hydrocarbon and ≧70% of CO, an NOx degradation of 26% is achieved. However, the reaction in the exhaust gas of an engine operated with a lean mixture has the disadvantage that the reaction of NOx with the fuel used as a reducing agent competes with its combustion in the oxygen excess present. For this reason, only a small part is effectively used for reducing the amount of NOx. The larger part of the fuel is lost without being used. The efficiency can be expressed chemically by the selectivity of the denox reaction. Frequently, the stoichiometric ratios of the denox reaction are neglected so that the selectivity data do not express the efficiency of the hydrocarbon directly. Thus, WO 98/40153 describes, for example, an NOx selectivity of from 0.3 to 1.0, which, when the stoichiometry is taken into account, would correspond to the use of from 1.5 to 5% of the available hydrocarbon propane for the denox reaction. The remaining hydrocarbon is directly combusted.
If these low efficiencies are applied to the NOx reduction with fuel, the theoretical amount of fuel to be used is so high that the fuel dose required for achieving the stipulated EURO IV exhaust gas standard overcompensates the advantageous consumption of the engine operated using a lean mixture. However, the fuel dose can be used neither economically nor technically in an expedient manner since the high heat of combustion of the fuel heats up the exhaust gas catalyst to such an extent that total combustion prevails. In any case, the competing total combustion of the reducing agent limits the NOx degradation so that the EURO IV standard cannot be achieved in this manner.
It is desirable to remove oxides of nitrogen reductively in the lean exhaust gas. Organic substances can be used as selective reducing agents for the catalytic removal of oxides of nitrogen from the exhaust gas of internal combustion engines. For example, in EP-A-0 537 942, the NOx reduction over highly acidic γ-Al2O3 with the aid of organic substances is described. The publication states that alkanes, alkenes, alkynes, aromatics, alcohols, aldehydes, ketones, ethers and esters can be used as such reducing agents. The publication furthermore describes the metering of these substances into the exhaust gas stream upstream of the NOx reduction catalyst via a nozzle. The nozzle physically atomizes, i.e. disperses, liquid or gaseous reactants. The NOx reduction with propylene as reducing agent is described in detail. The NOx degradation values of up to 80% are achieved only at temperatures above 500° C. This fact is disadvantageous since the temperature of the exhaust gas, in particular of diesel engines, is 100-400° C. and hence substantially below the NOx degradation temperature described in EP-A-0 537 942.
The reducing agents can be provided either by carrying in a separate tank or by on-board production from a precursor. The latter variant has advantages since the vehicle fuel can be used as the precursor so that there is also no need for an expensive on-board infrastructure for carrying the precursor. Thus, JP-A-112 44663 describes a process in which the hydrocarbon-containing exhaust gas is passed first over a partial oxidation catalyst and then over the actual NOx reduction catalyst. The partial oxidation catalysts described are titanium oxides which have been doped with various transition metals. Noble metal-containing active materials are used for the actual NOx reduction. The disadvantage of this process is the fact that the temperature of the partial oxidation catalyst is not constant since said catalyst is alternatively heated and cooled by the temperature of the exhaust gas stream. The temperature of the catalyst system and hence the denox performance thus depends on the operating point of the engine.
JP-A-100 005 46 describes a similar process. Here too, the hydrocarbon used for the NOx reduction is partially oxidized beforehand. For this purpose, at least a part of the exhaust gas or a separate air stream is mixed with hydrocarbon and an oxygen-containing, organic substance and this mixture is oxidized to aldehydes in an oxidation unit. The residence time of the gas stream in the oxidation reactor is set at 0.05-1 second. The aldehydes are passed into the main exhaust gas stream and reacted with the oxides of nitrogen contained therein over an NOx reduction catalyst. The active materials consist of various transition metals (Ag, Co, Ni) supported on γ-Al2O3. The disadvantage of these systems is that they exhibit NOx degradation only at temperatures above 350° C. It is true that the NOxdegradation temperature can be reduced considerably by using Pt/Ce as dopant. However, the optimum temperature for operating the oxidation unit is still very high at 500° C. Engine components suffer as a result of the considerable heat radiation of the oxidation unit, and the autothermal operation of this oxidation unit is not possible. Such a converter must be externally heated.
It is an object of the present invention to provide a process for the catalytic conversion of fuel and for removing oxides of nitrogen from exhaust gases of internal combustion engines, which process avoids the disadvantages of the existing processes.